Reactive sulfur species: kinetics and mechanism of the reaction of thiocarbamate-S-oxide with cysteine.
Hypothiocyanite (OSCN(-)) is a putative antimicrobial that is produced by defensive human peroxidases, including salivary peroxidase, lactoperoxidase, eosinophil peroxidase, and myeloperoxidase. The reaction of OSCN(-) with cysteine-derived sulfhydryl groups is believed to be involved in the antimicrobial mechanism of action. Hypothiocyanite decomposes via an unknown mechanism that involves multiple redox and hydrolytic steps to eventually yield mixtures of CN(-)/OCN(-) and SO(3)(2-)/SO(4)(2-). Until recently, no information was available regarding the chemical nature of the intermediate(s) that are produced during the chemical cascade, but we have shown that OSCN(-) undergoes hydrolysis to give the new compound thiocarbamate-S-oxide, H(2)NC(=O)SO(-). In the present paper, we demonstrate that H(2)NC(=O)SO(-) reacts with cysteine (CySH) via a two-step mechanism that is analogous to that of OSCN(-) with cysteine: HOZ + CySH --> CySZ + H(2)O and CySZ + CySH --> CySSCy + HZ, where Z = SCN and SC(=O)NH(2), respectively. The kinetics and mechanism of both steps of the reaction of H(2)NC(=O)SO(-) with CySH have been investigated as a function of pH.
Wang
Xiaoguang
X
Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, USA.
Ashby
Michael T
MT
D003545:Cysteine / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010087:Oxides / Q000737:chemistry; D013859:Thiocarbamates / Q000737:chemistry*; D013861:Thiocyanates / Q000737:chemistry
D010087:Oxides; D013859:Thiocarbamates; D013861:Thiocyanates; C014607:hypothiocyanite ion; D003545:Cysteine